Traditional Circuit Switched (CS) mobile telecommunication networks, such as a Public Land Mobile Network (PLMN) implemented as e.g. a Global System for Mobile communications (GSM) or Universal Mobile Telecommunication System (UMTS) do nowadays co-exist with Voice over Internet Protocol (VoIP) telecommunication networks.
IP telecommunication network technology is generally applied for new telecommunication networks and tends to be additional to, or eventually replacing, the traditional CS telecommunication networks.
However, as it seems not realistic to consider that all the traditional CS telecommunication networks are replaced by the PS telecommunication networks on a short term, e.g. for economical reasons regarding investments, it is expected that during a considerable period of time both CS and IP telecommunication networks will co-exist.
Although CS and IP technology differ considerably from a technical viewpoint, it is regarded as a requirement for telecommunication network operators that both CS and IP type telecommunication networks should co-operate, particularly in cases where a calling party and a called party have terminals that do not belong to the same, either CS or IP, telecommunication network type. IP telecommunication networks are an implementation of Packet Switched (PS) network technology.
Traditional CS telecommunication networks apply for call set-up an addressing technology according to standard E.164 as defined by the Telecommunication standardization sector of the International Telecommunication Union (ITU-T), where each terminal has been allocated a unique string of at most 15 digits. The PS network technology for telecommunication networks applies in general, e.g. for VoIP, an Internet Protocol (IP) address allocated to a terminal. These IP addresses, as an example, may look like 192.45.206.120 for an IP version 4 address format. Apart from different address formats, from technological viewpoint the CS and the PS telecommunication network technologies further differ in that at CS network technology a link is established by a circuit dedicated to a single connection between two endpoints, whereas at the PS network technology a link is established by a packet stream, routed via available connections between the endpoints wherein these connections are possibly shared with other links.
In spite of the differences between the CS and the PS telecommunication network types, prior art has provided methods to set up a connection from a terminal residing in the CS telecommunication network towards an terminal residing in the PS telecommunication network.
An example of such a solution is published patent application, EP 1 111 893 A2 that discloses a method and system where a user of a terminal, residing at a CS telecommunication network, calls a party having a terminal residing in a PS telecommunication network by means of calling an E.164 address allocated to the called terminal. This method with the allocated E.164 address being part of a phone numbering plan is a solution for addressing terminals having internet format addresses, wherein each terminal to be called, residing in a PS telecommunication network needs two addresses, an E.164 address as well as a PS network type address.
One skilled in the art will recognise that use of network resources explicitly arranged for setting up a call from a terminal residing the CS telecommunication network towards a terminal residing in the PS telecommunication network involves an extension of the common telecommunication network with at least another entity maintaining the relation between both addressing formats requiring additional network links and maintenance.
Furthermore telecommunication network operators should arrange for the E.164 phone numbering plan with a range of reserved phone numbers to be allocated to non-CS telecommunication network type terminals. This range out of the total range delimited by 15 digits cannot be used anymore for CS telecommunication network type terminals. The extension of the existing numbering plan based on the E.164 may thus eventually run out of available numbers.